Your computer knows what you did last weekend -- but that's okay because most of your other gadgets do, too. Your browser remembers your Web reading list, your cellphone saved your calls, and your MP3 player can recite the songs you heard.

And most of us seem content to have all this sentient machinery memorizing our daily routines, so long as all the data stay with us. A little surveillance of ourselves can be fine if we, and nobody else, get to see the results.

Your digital camera may be the next gadget to upgrade its self-awareness. It already records when you take photos, and now it can inform you where you shot them as well. You won't have to remember where you photographed each vacation shot; your photos will tell you.

This feat comes courtesy of a $129.99 device called the Eye-Fi Explore. It slips into a camera's SD card slot like any other memory unit, but this two-gigabyte card includes a WiFi receiver that connects to a database of wireless networks to determine the location of your pictures.

The Eye-Fi Explore doesn't do this everywhere and doesn't match the accuracy of a Global Positioning System receiver, but it's the simplest way to "geotag" your pictures.

The Explore, like two cheaper cards from Eye-Fi, of Mountain View, Calif., can also upload photos on the go. As long as it can log in to a WiFi network you've told it to remember (or if it grabs an open, unsecured hot spot), it can transfer your shots to a Mac or Windows computer, or any of more than 20 photo-sharing sites, moments after you've taken them.

Log on using the company's software, and you'll see your pictures alongside a Google Maps view of each shot.

Testing an Explore card in the District and on a trip to New York showed how this fusion of hardware, software and Web services can work -- or at least fail in a fascinating manner.

Photos taken in my house, on my walk to the Metro and between my subway stop and the office all showed up within feet of the correct locations, and Eye-Fi placed a shot from a train platform at Union Station a block or two away.

It was almost spooky to see my path plotted on the map like this.

The Explore also quickly uploaded these shots over my home and office wireless networks, though the transfers drained much of the camera's batteries and sometimes halted when the camera shut off on its own. In separate trials, the card deposited pictures into a Mac's iPhoto library, a Windows Vista PC's pictures folder, and Flickr and Photoshop Express accounts.

This card, however, did nothing with pictures from the train ride to New York, ignoring not only those snapped in rural stretches of Maryland but a few from just outside Philadelphia; Newark; and Trenton, N.J.

Once in New York, the Explore found its footing again. It put photos from a Midtown stretch of Park Avenue and the Brooklyn Heights abode of two friends within a few dozen yards of their actual locations and was almost as accurate with pictures taken outside the Harlem apartment of another couple.

But Eye-Fi placed shots from inside the Harlem apartment three miles south. The spot it pinpointed looked suspiciously familiar -- as it should have: Those friends lived in that location (on the Upper East Side) until late last year.

Explaining that coincidence, as well as the Explore's flubs on the train, requires some detail about how this card and its software cooperate to put your photos on the map.

The card first records the unique signatures of any nearby wireless networks -- the anonymous media access control (MAC) address encoded in every networking device. Once it uploads your photos, Eye-Fi runs those MAC addresses through a database of WiFi networks compiled by a Boston firm, Skyhook Wireless, that sends cars with WiFi and GPS sensors down one street after another through parts of North America, Europe and Asia.

The limits of Skyhook's database -- also employed by Apple's iPhone and iPod Touch -- restrict the reach of the Eye-Fi's geotagging. When it contains limited, out-of-date information -- say, if Skyhook's record of my friends' wireless network dated to their old apartment -- the system can give an incorrect identification.

And if the Explore can't sniff any wireless networks at all, its software has to give up. That alone rules out Eye-Fi's card for many trips, inasmuch as a basic reason to go on vacation is to escape the clutches of the Web.

For those travels, a camera with a Global Positioning System receiver might make a lot more sense but would cost too much and consume even more electricity.

And yet, the electronics industry can do an amazing job of cutting costs and extending battery life when it must. Someday, all digital photography will work like the best parts of my Eye-Fi experience -- and our cameras may remember more about our trips than we will.